drm/amdgpu: fix HMM config dependency issue

Only select HMM_MIRROR will get kernel config dependency warnings
if CONFIG_HMM is missing in the config. Add depends on HMM will
solve the issue.

Add conditional compilation to fix compilation errors if HMM_MIRROR
is not enabled as HMM config is not enabled.

Remove unused function amdgpu_ttm_tt_mark_user_pages.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>